1. Field of the Invention
This invention relates to a method of forming a film for a semiconductor device.
2. Description of the Related Art
A lift off method has been used for patterning of a thin film to which etching is difficult. For example, an electrode film of a transistor is formed by the lift off method. When forming the electrode film by the lift off method, a resist (or a metal) is applied to a substrate surface to form a resist pattern having a shape opposite to that of the electrode film. Then, a material of the electrode film is applied onto the resist pattern by vapor deposition or sputtering. Then, an electrode material film deposited on the resist film is removed together with the resist film. The remainder of the removed electrode material film becomes the electrode film.
In the lift off process, the resist film is formed such that it has a vertical sectional shape of a reverse-trapezoid. That is, the resist film is patterned such that the upper side thereof has a width larger than that of the lower side thereof. When the resist film has such a shape, unnecessary part of the electrode material film on the resist film can be separated, for removal, from necessary part of the electrode material film, which becomes the final electrode film. The pattern forming by the lift off method is disclosed in Japanese Patent Application Kokai Number 6-104256.
As the integration of a semiconductor device advances, the distance between drain and source electrodes of a transistor has been reduced. For example, when forming drain and source electrodes with the distance therebetween of 2 μm, the width of the upper surface of the resist film formed between the regions of both the electrodes is also made approximately 2 μm in the manufacturing process. As described above, the resist film for the lift off process has the shape of reverse-trapezoid so that the width of the lower surface thereof must be made smaller than that of the upper surface (for example, 1.3 μm or less) if formed in accordance with the conventional method.
However, when the width of the lower surface is made small, it is possible that the resist film slopes and collapses in the worst case. If the resist film slopes, the drain and source electrodes cannot be formed at predetermined positions with predetermined shapes, causing adverse effects on the quality of the transistor.